An organic electroluminescent device (hereinafter abbreviated as “organic EL device”) using an organic substance is a promising solid-state emitting type inexpensive and large full-color display device. Accordingly, the organic EL device has been extensively developed. In-general, an organic EL device includes an emitting layer and a pair of opposing electrodes holding the emitting layer therebetween.
When an electric field is applied between the electrodes, electrons are injected from the cathode and holes are injected from the anode. The electrons recombine with the holes in the emitting layer to produce an excited state, and energy is emitted as light when the excited state returns to the ground state.
Various organic EL devices have been known. For example, patent document 1 discloses an organic EL device having a device configuration of indium tin oxide (ITO)/hole transporting layer/emitting layer/cathode, in which an aromatic tertiary amine is used as the material for the hole transporting layer. This device configuration achieves a high luminance of several hundred cd/m2 at an applied voltage of 20 V or less. It has been reported that a luminous efficiency of about 40 lm/W or more is achieved at a luminance equal to or less than several thousand cd/m2 by using an iridium complex (phosphorescent dopant) for the emitting layer (non-patent document 1).
Patent document 2 discloses a highly efficient organic EL device utilizing the migration of plural visible luminescent dopants. Patent documents 3 and 4 disclose red fluorescent EL devices using Irppy as an auxiliary dopant.
Non-patent document 2 discloses an organic EL device having a host:dopant complex two-layer configuration using two host materials and two dopant complexes (e.g. trisphenylpyrazolediridium (Irsppzd3)). In the non-patent document 2, the dopant complexes have a luminous peak of 450 nm or more.
Patent document 5 discloses an organic EL device using two types of dopants in the emitting layer. In the patent document 5, the dopants are each phosphorescent metal complexes of which the ligand is an organic compound. One of the two types of dopants with a longer maximum emission wavelength has a substituent in the ligand structure, and is contained in the emitting layer at a concentration lower than that of the other dopant.
Patent document 6 discloses technology which limits the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of a host and a phosphorescent dopant contained in the emitting layer. The patent document 6 discloses an example in which a dopant is a complex which emits light in the visible region, and one dopant is contained in the emitting layer.    [Patent document 1] JP-A-63-295695    [Patent document 2] JP-A-2004-319456    [Patent document 3] JP-T-2003-520391    [Patent document 4] JP-T-2004-526284    [Patent document 5] JP-A-2005-100957    [Patent document 6] JP-A-2005-203293    [Non-patent document 1] T. Tsutsui et. al., Jpn. J. Appl. Phys. Vol. 38 (1999), pp. L1502 to L1504    [Non-patent document 2] Appl. Phys. Lett. 86, 263502 (2005)
Since most of these phosphorescent organic EL devices emit green or red light, an increase in the number of colors and particularly an increase in blue luminous efficiency have been demanded. In particular, a blue device configuration with a luminous quantum efficiency of 5% or more is rare.
When applying an organic EL device to a flat panel display or the like, the organic EL device is required to exhibit an improved luminous efficiency and reduced power consumption. However, the above device configuration has a disadvantage in that the luminous efficiency significantly decreases accompanying an increase in the luminance. Therefore, it is difficult to reduce the power consumption of the flat panel display.
An object of the invention is to develop a blue organic EL device exhibiting a high current efficiency or a high luminous efficiency.